Valve module for an air braking system of a heavy vehicle

ABSTRACT

A valve module is provided for enabling a vehicle to control an autonomous event of the vehicle. The valve module comprises a relay valve, a first solenoid valve, and a second solenoid valve. A first control pressure can be delivered through the first solenoid valve and applied to a control port of the relay valve. In one embodiment, a second control pressure can be delivered through the second solenoid valve and combined with the first control pressure. The combined first and second control pressures are applied to the control port of the relay valve. In another embodiment, a second control pressure can be delivered through the second solenoid valve only when no first control pressure is delivered through the first solenoid valve.

BACKGROUND

The present application relates to heavy vehicle air braking systems,and is particularly directed to a valve module for an air braking systemof a heavy vehicle, such as a truck.

A typical air braking system includes an anti-lock brake system (ABS)that has a number of ABS modulator valves. The ABS modulator valves arecontrolled in response to electrical control signals from an ABScontroller to modulate air flow to service brake chambers to preventlocking of braked wheels to improve the braking characteristics of theheavy vehicle. The ABS modulator valves in some heavy vehicles arecontrolled to control not only an anti-lock braking event, but also tocontrol other types of autonomous braking events that are auxiliary tothe normal service braking function of the heavy vehicle. An exampleautonomous braking event that is auxiliary to the normal service brakingfunction comprises hill start assist (HSA). HSA prevents rolling back ofthe heavy vehicle when the vehicle is stationary on an uphill inclineand the foot of the vehicle driver is transitioning from the vehiclefoot brake pedal to the vehicle foot accelerator pedal to accelerate thevehicle from its stationary position on the uphill incline. It would bedesirable to improve performance of autonomous braking events, such asHSA, in heavy vehicles.

SUMMARY

In accordance with an embodiment, a valve module is provided for avehicle having a compressed air supply, a driver demand device forproviding a driver demand pressure indicative of driver intent to applybrakes, one or more brake chambers, and a controller for controllingdelivery of air flow through the valve module to the one or more brakechambers to control an autonomous braking event of the vehicle. Thevalve module comprises a relay valve having a control port, a supplyport, and a delivery port, wherein (i) the supply port of the relayvalve is connectable in fluid communication with the compressed airsupply, and (ii) the delivery port of the relay valve is connectable influid communication with the one or more brake chambers. The valvemodule further comprises a first solenoid valve having a first solenoid,at least one supply port, and a delivery port connected in fluidcommunication with the control port of the relay valve, wherein thefirst solenoid is responsive to a first electrical signal from thecontroller to deliver a first control pressure from the driver demanddevice through the first solenoid valve to the control port of the relayvalve to control air flow from the supply port of the relay valvethrough the relay valve to the delivery port of the relay valve tocontrol air flow to the one or more brake chambers. The valve modulealso comprises a second solenoid valve having a second solenoid, atleast one supply port, and a delivery port, wherein (i) the secondsolenoid is responsive to a second electrical signal from the controllerto deliver a second control pressure to the first solenoid valve tocombine with the first control pressure, and (ii) the combined first andsecond control pressures are applied to the control port of the relayvalve to control air flow from the supply port of the relay valvethrough the relay valve to the delivery port of the relay valve tocontrol air flow to the one or more brake chambers and thereby tocontrol the autonomous braking event of the vehicle.

In accordance with another embodiment, a valve module is provided for avehicle having a compressed air supply, a driver demand device forproviding a driver demand pressure indicative of driver intent to applybrakes, one or more brake chambers, and a controller for controllingdelivery of air flow through the valve module to the one or more brakechambers to control an autonomous braking event of the vehicle. Thevalve module comprises a relay valve having a control port, a supplyport, and a delivery port, wherein (i) the supply port of the relayvalve is connectable in fluid communication with the compressed airsupply, and (ii) the delivery port of the relay valve is connectable influid communication with the one or more brake chambers. The valvemodule further comprise a first solenoid valve having a first solenoid,a supply port, and a delivery port connected in fluid communication withthe control port of the relay valve. The valve module also comprises asecond solenoid valve having a second solenoid, a supply port, and adelivery port connected in fluid communication with the control port ofthe relay valve, wherein (i) the first solenoid is responsive to a firstelectrical signal from the controller to deliver a first controlpressure from the driver demand device to the supply port of the firstsolenoid valve through the first solenoid valve to the control port ofthe relay valve to control air flow from the supply port of the relayvalve through the relay valve to the delivery port of the relay valve tocontrol air flow to the one or more brake chambers and thereby tocontrol the autonomous braking event of the vehicle, and (ii) the secondsolenoid is responsive to a second electrical signal from the controllerto deliver a second control pressure from the compressed air supply tothe supply port of the second solenoid valve through the second solenoidvalve to the control port of the relay valve only when no first controlpressure is being delivered from the driver demand device through thefirst solenoid valve to the control port of the relay valve.

In accordance with still another embodiment, a valve module is providedfor a vehicle having a compressed air supply, a driver demand device forproviding a driver demand pressure indicative of driver intent to applybrakes, one or more brake chambers, and a controller for controllingdelivery of air flow through the valve module to the one or more brakechambers to control an autonomous braking event of the vehicle. Thevalve module comprises a relay valve having a control port, a supplyport, and a delivery port, wherein (i) the supply port of the relayvalve is connectable in fluid communication with the compressed airsupply, and (ii) the delivery port of the relay valve is connectable influid communication with the one or more brake chambers. The valvemodule further comprises a one-way check valve having an inlet portconnectable in fluid communication with the driver demand device, and anoutlet port. The valve module also comprises a first solenoid valvehaving a first supply port, a second supply port, a delivery port, and asolenoid that is controllable in response to a first electrical signalfrom the controller, wherein (i) the first supply port of the firstsolenoid valve is connected in fluid communication with the driverdemand device, and (ii) the delivery port of the first solenoid valve isconnected in fluid communication with the control port of the relayvalve. The valve module further comprises a second solenoid valve havinga first supply port, a second supply port, a delivery port, and asolenoid that is controllable in response to a second electrical signalfrom the controller, wherein (i) the first supply port of the secondsolenoid valve is connectable in fluid communication with the compressedair supply, (ii) the second supply port of the second solenoid valve isconnected in fluid communication with atmosphere, and (iii) the deliveryport of the second solenoid valve is connected in fluid communicationwith the second supply port of the first solenoid valve.

In accordance with yet another embodiment, a valve module is providedfor a vehicle having a compressed air supply, a driver demand device forproviding a driver demand input pressure indicative of driver intent toapply brakes, one or more brake chambers, and a controller forcontrolling delivery of air flow through the valve module to the one ormore brake chambers to control an autonomous braking event of thevehicle. The valve module comprises a relay valve having a control port,a supply port, and a delivery port, wherein (i) the supply port of therelay valve is connectable in fluid communication with the compressedair supply, and (ii) the delivery port of the relay valve is connectablein fluid communication with the one or more brake chambers. The valvemodule further comprises a first solenoid valve having a supply port, adelivery port, and a solenoid that is controllable in response to afirst electrical signal from the controller, wherein (i) the supply portof the first solenoid valve is connectable in fluid communication withdriver demand device, and (ii) the delivery port of the first solenoidvalve is connected in fluid communication with the control port of therelay valve. The valve module also comprises a second solenoid valvehaving a supply port, a delivery port, and a solenoid that iscontrollable in response to a second electrical signal from thecontroller, wherein (i) the supply port of the second solenoid valve isconnectable in fluid communication with the compressed air supply, and(ii) the delivery port of the second solenoid valve is connected influid communication with the control port of the relay valve and thedelivery port of the first solenoid valve. The valve module furthercomprises a one-way check valve having an inlet port connected in fluidcommunication with the supply port of the first solenoid valve and anoutlet port connected in fluid communication with the control port ofthe relay valve and the delivery ports of the first and second solenoidvalves.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a heavy vehicle air brakingsystem embodying a valve module constructed in accordance with anembodiment.

FIG. 2 is a schematic block diagram of the valve module of FIG. 1 withparts in another position.

FIG. 3 is a schematic block diagram of the valve module of FIG. 2 withparts in yet another position.

FIG. 4 is a schematic block diagram of the valve module of FIG. 3 withparts in still another position.

FIG. 5 is a flow diagram depicting a control method for the valve moduleof FIGS. 1-4 in accordance with an embodiment.

FIG. 6 is a schematic block diagram of a heavy vehicle air brakingsystem embodying a valve module constructed in accordance with anotherembodiment.

FIG. 7 is a schematic block diagram of the valve module of FIG. 6 withparts in another position.

FIG. 8 is a schematic block diagram of the valve module of FIG. 7 withparts in yet another position.

FIG. 9 is a flow diagram depicting a control method for the valve moduleof FIGS. 6-8 in accordance with an embodiment.

DETAILED DESCRIPTION

Referring to FIG. 1, a schematic block diagram of valve module 100constructed in accordance with an embodiment is illustrated. Valvemodule 100 is embodied in a heavy vehicle, such as a truck, that has apneumatic braking system and is equipped with an anti-lock brakingsystem (ABS). In FIG. 1, electrical line connections are shown as solidlines, and pneumatic lines connections are shown as dashed lines.

The braking system includes controller 110 that responds to signal online 112 from foot brake transducer 114, one or more signals on line 116from one or more other controllers 118, and one or more signals on line120 from one or more wheel speed sensors 122. Controller 110 may alsocontrol ABS or may be dedicated to the operation of valve module 100.

Foot brake transducer 114 provides signal on line 112 in response to thevehicle driver depressing a foot brake pedal (not shown). Signal on line112 is indicative of the vehicle driver's intent to apply service brakesof the vehicle. Structure and operation of the foot brake pedal and footbrake transducer 114 are known and, therefore, will not be described.

One or more other controllers 118 may include a transmission controller,for example, that provides signal on line 116 indicative of status ofthe vehicle transmission. Other types of controllers providing one ormore signals on line 116 are possible. One or more wheel speed sensors122 may be any type. Structure and operation of the wheel speed sensorsare known and, therefore, will not be described.

The braking system further includes a driver demand device in the formof foot brake valve 124 that may comprise any type of commerciallyavailable foot brake valve. Foot brake pedal (not shown) may be part offoot brake valve 124, or may be separate from foot brake valve 124. Footbrake valve 124 provides pneumatic control signal in line 126 inresponse to the vehicle driver depressing the foot brake pedal.Structure and operation of foot brake valves and foot brake pedals areknown and, therefore, will not be described. Pneumatic control signal inline 126 is connectable in fluid communication with air inlet port 162of valve module 100.

Although the driver demand device in FIG. 1 is shown in the form of afoot-operated brake valve, it is conceivable that driver demand devicemay be in a different form. For example, the driver demand device may bein the form of a hand-operated brake valve. Other types of driver demanddevices are possible.

The braking system also includes compressed air supply 128 that providesa source of compressed air in line 130. Compressed air in line 130 isconnectable in fluid communication to air inlet port 158 of valve module100.

Controller 110 provides electrical control signals on lines 132, 134that are provided to valve module 100. Valve module 100 responds toelectrical control signals on lines 132, 134 and pneumatic controlsignal in line 126 to control delivery of compressed air in line 130 toair outlet port 160 of valve module 100. Air outlet port 160 of valvemodule 100 is connectable in fluid communication with line 136. Line 136is connected in fluid communication with one or more ABS modulatorvalves 140. Pressure sensor 138 may be connected in fluid communicationwith line 136 to monitor air pressure in line 136.

One or more ABS modulator valves 140 may be operatively connected to oneor more service brake chambers 144 in known manner. When not activated,each of the one or more ABS modulator valves 140 allows air to passdirectly therethrough. When activated, each of the one or more ABSmodulator valves 140 modulate compressed air in line 136 to delivermodulated compressed air to the one or more service brake chambers 144.The number of service brake chambers depends on the number of axles ofthe particular vehicle. Accordingly, the number of ABS modulator valvesand the number of pneumatic lines depends on the number of axles of theparticular vehicle. Structure and operation of ABS modulator valves tocontrol service brake chambers are known and, therefore, will not bedescribed.

Valve module 100 responds to electrical control signals on lines 132,134 and pneumatic control signal on line 126 to control delivery ofcompressed air flow from line 130 through valve module 100 to line 136to control operation of the one or more service brake chambers 144, aswill be described in detail hereinbelow.

Valve module 100 includes relay valve 150 having control port 152,supply port 154, and delivery port 156. Supply port 154 of relay valve150 is connected in fluid communication in line 155 with air inlet port158 that is connectable in fluid communication with line 130 fromcompressed air supply 128. Delivery port 156 of relay valve 150 isconnected in line 157 with air outlet port 160 that is connectable influid communication with line 136 to the one or more ABS modulators 140.

Valve module 100 further includes one-way check valve 190 having inletport 192 and outlet port 194. Inlet port 192 of one-way check valve 190is connected in fluid communication in line 193 with air inlet port 162of valve module 100. Outlet port 194 of one-way check valve 190 isconnected in fluid communication in line 195 with first solenoid valve170 of valve module 100.

First solenoid valve 170 includes first supply port 171, second supplyport 172, delivery port 174, and activatable solenoid 176 that iscontrollable in response to electrical control signal on line 132 fromcontroller 110. First supply port 171 of first solenoid valve 170 isconnected in fluid communication in line 195 with output port 194 ofone-way check valve 190. Delivery port 174 of first solenoid valve 170is connected in fluid communication in line 175 with control port 152 ofrelay valve 150. First solenoid valve 170 has orifice 177 that will bedescribed in detail later.

Valve module 100 further includes second solenoid valve 180 having firstsupply port 181, second supply port 182, delivery port 184, andactivatable solenoid 186 that is controllable in response to electricalcontrol signal on line 134 from controller 110. First supply port 181 ofsecond solenoid valve 180 is connected in line 189 to line 155 to airinlet port 158 that is connectable in fluid communication in line 130with compressed air supply 128. Second supply port 182 of secondsolenoid valve 180 is connected in fluid communication with atmosphereand acts as an exhaust port. Delivery port 184 of second solenoid valve180 is connected in fluid communication in line 185 with second supplyport 172 of first solenoid valve 170.

First solenoid valve 170 may comprise an activatable 3/2 valve. Acombination of first solenoid valve 170 and relay valve 150 shown inFIG. 1 may comprise Model No. ATR-6™ Valve commercially available fromBendix Commercial Vehicle Systems LLC located in Elyria, Ohio. Whenfirst solenoid valve 170 is activated in response to electrical controlsignal on line 132 from controller 110, first solenoid valve 170 movesfrom a first position as shown in FIG. 1 to a second position as shownin FIG. 2. In the first position of first solenoid valve 170 shown inFIG. 1, air is blocked from flowing through first supply port 171 offirst solenoid valve 170 and air is allowed to flow from second supplyport 172 of first solenoid valve 170 through first solenoid valve 170 tocontrol port 152 of relay valve 150. In the second position of firstsolenoid valve 170 shown in FIG. 2, air is allowed to flow from bothfirst and second supply ports 171, 172 through first solenoid valve 170to control port 152 of relay valve 150. Spring 178 of first solenoidvalve 170 returns first solenoid valve 170 from the second positionshown in FIG. 2 back to the first position shown in FIG. 1 when solenoid176 of first solenoid valve 170 is deactivated.

Second solenoid valve 180 may comprise an activatable 3/2 valve, such asModel No. AT-3™ Valve commercially available from Bendix CommercialVehicle Systems LLC located in Elyria, Ohio. When second solenoid valve180 is activated in response to electrical control signal on line 134from controller 110, second solenoid valve 180 moves from a firstposition as shown in FIG. 2 to a second position as shown in FIG. 3. Inthe first position of second solenoid valve 180 shown in FIG. 2, air isblocked from flowing through first supply port 181 of second solenoidvalve 180 and air can be exhausted through second supply port 182. Inthe second position of second solenoid valve 180 shown in FIG. 3, air isallowed to flow from first supply port 181 of second solenoid valve 180through second solenoid valve 180 to second supply port 172 of firstsolenoid valve 170. Spring 188 of second solenoid valve 180 returnssecond solenoid valve 180 from the second position shown in FIG. 3 backto the first position shown in FIG. 2 when solenoid 186 of secondsolenoid valve 180 is deactivated.

Relay valve 150 may comprise an air-operated, graduating directionalcontrol valve of high capacity and fast response. Relay valve 150graduates, holds, and releases air pressure from brake chambers to whichit is connectable. Relay valve 150 may comprise Model No. R-12® Valvecommercially available from Bendix Commercial Vehicle Systems LLClocated in Elyria, Ohio.

One-way check valve 190 may comprise any of a variety of types. Forexample, one-way check valve 190 may comprise Model No. SC-3™ Valvecommercially available from Bendix Commercial Vehicle Systems LLClocated in Elyria, Ohio. Output port 194 of one-way check valve 190 isconnected in fluid communication with first solenoid valve 170 such thatone-way check valve 190 is effectively connected in series with firstsolenoid valve 170.

Although the above description describes relay valve 150, first solenoidvalve 170, second solenoid valve 180, and one-way check valve 190 asseparate components within valve module 100, it is conceivable that anycombination of these components may be integrated as a single physicalunit.

The arrangement of valve module 100 in the braking system shown in FIG.1 enables air to be controlled and delivered to the one or more servicebrake chambers 144 to affect an autonomous braking event that isauxiliary to the normal service braking function of the vehicle. Exampleautonomous braking events include hill start assist (HSA), tractioncontrol, electronic stability control, autonomous cruise control (ACC),and collision mitigation. Other types of autonomous braking events arepossible. An autonomous braking event is generally any event where acontroller (such as controller 110 in FIG. 1) is controlling componentsto increase pressure (or maintain pressure in the case of an HSA event)at control port 152 of relay valve 150 without any further driver input(or the driver removes foot from the foot pedal in the case of an HSAevent). For purposes of description, only an HSA event is described indetail hereinbelow.

HSA prevents rolling back of the vehicle when the vehicle is stationaryon an uphill incline and the foot of the vehicle driver is transitioningfrom the foot brake pedal to the foot accelerator pedal to acceleratethe vehicle from its stationary position on the uphill incline.

When the vehicle driver initially depresses the foot brake pedal to stopthe vehicle on an uphill incline, first solenoid valve 170 is in itsfirst position shown in FIG. 1, and second solenoid valve 180 is also inits first position in FIG. 1. In the first position of first solenoidvalve 170 shown in FIG. 1, air is blocked from flowing in line 195through first solenoid valve 170 to line 175 to control port 152 ofrelay valve 150. Any air pressure in line 175 is exhausted from deliveryport 174 through first solenoid valve 170 to second supply port 172 toline 185 to delivery port 184 through second solenoid valve 180 tosecond supply port 182 to atmosphere.

When controller 110 receives a combination of signals including signalon line 112 from foot brake transducer 114, signal on line 116 from oneor more other controllers 118, and signal on line 120 from one or morewheel sensors 122 calling for HSA to be initiated, controller 110provides electrical control signal on line 132 to activate solenoid 176of first solenoid valve 170. When solenoid 176 activates, first solenoidvalve 170 moves from its first position shown in FIG. 1 to its secondposition shown in FIG. 2.

In the second position of first solenoid valve 170 shown in FIG. 2, thecontrol air pressure previously applied to control port 152 of relay 150is “trapped” to hold the pressure in the one or more service chambers144 and thereby to maintain application of the service brakes of thevehicle during HSA. More specifically, one-way check valve 190 operatesto trap air pressure in line 195 and line 175 to control port 152 ofrelay valve 150.

When first solenoid valve 170 are in its second position shown in FIG.2, the vehicle driver can increase the air pressure at control port 152of relay valve 150 by depressing the foot brake pedal. When the vehicledriver depresses the foot brake pedal, the pressure in line 195increases, and is passed through first solenoid valve 170 to line 175 tocontrol port 152 of relay valve 150. The pressure applied to controlport 152 of relay valve 150 is trapped, and is a driver demand pressure.The trapped air pressure in line 195 and line 175 provides HSA, and isnot exhausted until first solenoid valve 170 is deactivated and returnsto its first position shown in FIG. 1.

When first solenoid valve 170 is activated as shown in its secondposition in FIG. 2, second solenoid valve 180 is activatable to movefrom its first position shown in FIG. 2 to its second position shown inFIG. 3 to autonomously increase control pressure to control port 152 ofrelay valve 150. More specifically, when second solenoid valve 180 isactivated and moves from its first position shown in FIG. 2 to itssecond position shown in FIG. 3, control pressure from compressed airsupply 128 passes through second solenoid valve 180 which, in turn,passes through first solenoid valve 170 to combine with control airpressure of first solenoid valve 170 to increase control pressure tocontrol port 152 of relay valve 150.

Second solenoid valve 180 can be activated after first solenoid valve170 has been activated. For example, second solenoid valve 180 can beactivated when a small air leak downstream from orifice 177 of firstsolenoid valve 170 occurs. Orifice 177 located in first solenoid valve170 is provided to reduce the chance of unintentionally applying theservice brakes of the vehicle in the event that there is a small airleak downstream from orifice 177, such as a small leak in relay valve150. Orifice 177 allows the air in relay valve 150 to leak out fasterthan compressed air from compressed air supply 128 can be suppliedthrough first and second solenoid valves 170, 180 to relay valve 150.

Also, second solenoid valve 180 can be activated without first solenoidvalve 170 being activated. In this case, second solenoid valve 180 movesfrom its first position shown in FIG. 1 to its second position shown inFIG. 4. When second solenoid valve 180 is activated and first solenoidvalve 170 is not activated, as shown in FIG. 4, full air pressure isbeing delivered from compressed air supply 128 through second solenoidvalve 180 and first solenoid valve 170 to relay valve 150.

Referring to FIG. 5, flow diagram 500 depicts a control method for valvemodule 100 of FIGS. 1-4 in accordance with an embodiment. In block 502,a determination is made as to whether an autonomous braking event isdesired. If determination in block 502 is negative (i.e., an autonomousbraking is not desired), the process proceed to block 504. In block 504,both first solenoid valve 170 and second solenoid valve 180 remaindeactivated as shown in their positions in FIG. 1. No control pressureis applied to control port 152 of relay valve 150. However, ifdetermination in block 502 is affirmative (i.e., an autonomous brakingevent is desired), the process proceeds to block 506.

In block 506, first solenoid valve 170 is activated and moves from itsfirst position shown in FIG. 1 to its second position shown in FIG. 2. Adetermination is then made in block 508 as to whether HSA is requested.If determination in block 508 is negative (i.e., HSA is not requested),the process proceeds to block 510. In block 510, second solenoid 180 isactivated and moves from its first position shown in FIG. 2 to itssecond position shown in FIG. 3. Then, in block 512, air supply pressurefrom compressed air supply 128 is applied through second solenoid valve180 and first solenoid valve 170 to control port 152 of relay valve 150.The control pressure at control port 152 rises to a pressure “P2”. Theprocess then proceeds to block 514.

In block 514, a determination is made as to whether the autonomousbraking event has ended. If the determination in block is negative(i.e., the autonomous braking event has not ended), the process returnsback to block 512. However, if the determination in block is affirmative(i.e., the autonomous braking event has ended), the process proceeds toblock 516. In block 516, second solenoid valve 180 is deactivated andmoves from its second position shown in FIG. 3 back to its firstposition shown in FIG. 2, and the control pressure at control port 152of relay valve 150 is exhausted through first solenoid valve 170 andsecond solenoid valve 180 to atmosphere at second supply port 182 ofsecond solenoid valve 182.

However, if the determination back in block 508 is affirmative (i.e.,HSA is requested), the process proceeds to block 520. A determination ismade in block 520 as to whether the control pressure that is beingapplied to control port 152 of relay valve 150 is at a pressure “P1”.The pressure “P1” is a driver demand input pressure received from footbrake valve 124).

The pressure “P1” is set when the driver puts foot on the foot brakepedal and activates foot brake valve 124. The pressure “P1” is appliedas the control pressure to control port 152 of relay valve 150. One-waycheck valve 190 allows the control pressure to control port 152 of relayvalve 150 to increase if the driver demand pressure “P1” from foot brakevalve 124 increases.

If determination in block 520 is negative (i.e., the control pressurethat is being applied to control port 152 of relay valve 150 is not atthe driver demand pressure “P1”), the process proceeds to block 540. Inblock 540, second solenoid valve 180 is activated and moves from itsfirst position shown in FIG. 2 to its second position shown in FIG. 3.When this occurs, the control pressure applied to control port 152 ofrelay valve 150 rises as shown in block 542 due to air supply pressurefrom compressed air supply 128 through second solenoid valve 180. Theprocess proceeds to block 544 in which a determination is made as towhether the control pressure being applied to control port 152 of relayvalve 150 has reached the driver demand pressure “P1”.

If the determination in block 544 is negative (i.e., the controlpressure applied to control port 152 of relay 150 has not reached thedriver demand pressure “P1”), then the process returns back to block 540and block 542 until the control pressure applied to control port 152 ofrelay 150 reaches the driver demand pressure “P1”. When thedetermination is affirmative (i.e., the control pressure at control port152 of relay valve 150 has reached the driver demand pressure “P1”), theprocess proceeds to block 546. In block 546, second solenoid valve 180is deactivated and moves from its second position shown in FIG. 3 backto its first position shown in FIG. 2. The process then proceeds toblock 528.

However, if the determination back in block 520 is affirmative (i.e.,the control pressure that is being applied to control port 152 of relayvalve 150 is at the driver demand pressure “P1”), the process proceedsto block 522. A determination is made in block 522 as to whether thecurrent driver demand pressure being received from foot brake valve 124is either maintained at pressure “P1” or has dropped below pressure“P1”.

If determination in block 522 is negative (i.e., the current driverdemand pressure being received from foot brake valve 124 is higher thanthe pressure “P1”), then the process proceeds to block 524. In block524, the control pressure applied to control port 152 of relay valve 150increases to the current driver input pressure being received from footbrake valve 124 via operation of one-way check valve 190. Morespecifically, after first solenoid valve 170 has been activated, one-waycheck valve 190 allows for increase of the control pressure that isbeing applied to control port 152 of relay valve 150 in response to anincrease of driver demand pressure at the driver demand device. Theprocess then proceeds to block 528.

However, if the determination back in block 522 is affirmative (i.e.,the current driver demand pressure being received from foot brake valve124 is either less than or equal to the pressure “P1”), the processproceed to block 526. In block 526, the pressure “P1” applied to controlport 152 of relay valve 150 is maintained and remains at the pressure“P1”. The process then proceeds to block 528.

In block 528, a determination is made as to whether the HSA has ended.HSA can end by either a timeout (three seconds from the activation ofthe HSA, for example) or the vehicle accelerating, for examples. If thedetermination in block 528 is negative (i.e., HSA has not ended), thenthe process returns back to block 520 to continue effecting HSA.However, if the determination in block 528 is affirmative (i.e., HSA hasended), then the process proceeds to block 530. In block 530, firstsolenoid valve 170 is deactivated and moves from its second positionshown in FIG. 2 back to its first position shown in FIG. 1 such that thecontrol pressure at control port 152 of relay valve 150 exhausts throughfirst solenoid valve 170 and second solenoid valve 180 to second supplyport 182 of second solenoid valve 180 to atmosphere. The process thenends.

Referring to FIG. 6, a schematic block diagram of valve module 200constructed in accordance with an embodiment is illustrated. Valvemodule 200 is embodied in a heavy vehicle, such as a truck, that has apneumatic braking system and is equipped with an anti-lock brakingsystem (ABS). In FIG. 6, electrical line connections are shown as solidlines, and pneumatic lines connections are shown as dashed lines.

The braking system includes controller 210 that responds to signal online 212 from foot brake transducer 214, one or more signals on line 216from one or more other controllers 218, and one or more signals on line220 from one or more wheel speed sensors 222. Controller 210 may alsocontrol ABS or may be dedicated to the operation of valve module 200.

Foot brake transducer 214 provides signal on line 212 in response to thevehicle driver depressing a foot brake pedal (not shown). Signal on line212 is indicative of the vehicle driver's intent to apply service brakesof the vehicle. Structure and operation of the foot brake pedal and footbrake transducer 214 are known and, therefore, will not be described.

One or more other controllers 218 may include a transmission controller,for example, that provides signal on line 216 indicative of status ofthe vehicle transmission. Other types of controllers providing one ormore signals on line 216 are possible. One or more wheel speed sensors222 may be any type. Structure and operation of the wheel speed sensorsare known and, therefore, will not be described.

The braking system further includes a driver demand device in the formof foot brake valve 224 that may comprise any type of commerciallyavailable foot brake valve. Foot brake pedal may be part of foot brakevalve 224, or may be separate from foot brake valve 224. Foot brakevalve 224 provides pneumatic control signal in line 226 in response tothe vehicle driver depressing the foot brake pedal. Structure andoperation of foot brake valves and foot brake pedals are known and,therefore, will not be described. Pneumatic control signal in line 226is connectable in fluid communication with air inlet port 262 of valvemodule 200.

Although the driver demand device in FIG. 6 is shown in the form of afoot-operated brake valve, it is conceivable that driver demand devicemay be in a different form. For example, the driver demand device may bein the form of a hand-operated brake valve. Other types of driver demanddevices are possible.

The braking system also includes compressed air supply 228 that providesa source of compressed air in line 230. Compressed air in line 230 isconnectable in fluid communication to air inlet port 258 of valve module200.

Controller 210 provides electrical control signals on lines 232, 234that are provided to valve module 200. Valve module 200 responds toelectrical control signals on lines 232, 234 and pneumatic controlsignal in line 226 to control delivery of compressed air in line 230 toair outlet port 260 of valve module 200. Air outlet port 260 of valvemodule 200 is connectable in fluid communication with line 236. Line 236is connected in fluid communication with the one or more ABS modulatorvalves 240. Pressure sensor 238 may be connected in fluid communicationwith line 236 to monitor air pressure in line 236.

One or more ABS modulator valves 240 may be operatively connected to oneor more service brake chambers 244 in known manner. When not activated,each of the one or more ABS modulator valves 240 allows air to passdirectly therethrough. When activated, each of the one or more ABSmodulator valves 240 modulate compressed air in line 236 to delivermodulated compressed air to the one or more service brake chambers 244.The number of service brake chambers depends on the number of axles ofthe particular vehicle. Accordingly, the number of ABS modulator valvesand the number of pneumatic lines depends on the number of axles of theparticular vehicle. Structure and operation of ABS modulator valves tocontrol service brake chambers are known and, therefore, will not bedescribed.

Valve module 200 responds to electrical control signals on lines 232,234 and pneumatic control signal on line 226 to control delivery ofcompressed air flow from line 230 through valve module 200 to line 236to control operation of the one or more service brake chambers 244, aswill be described in detail hereinbelow.

Valve module 200 includes relay valve 250 having control port 252,supply port 254, and delivery port 256. Supply port 254 of relay valve250 is connected in fluid communication in line 255 with air inlet port258 that is connectable in fluid communication with line 230 fromcompressed air supply 228. Delivery port 256 of relay valve 250 isconnected in line 257 with air outlet port 260 that is connectable influid communication with line 236 to the one or more ABS modulatorvalves 240.

Valve module 200 further includes first solenoid valve 270 having supplyport 271, delivery port 274, and activatable solenoid 276 that iscontrollable in response to electrical control signal on line 232 fromcontroller 210. Supply port 271 of first solenoid valve 270 is connectedin line 295 to air inlet port 262 that is connectable in fluidcommunication in line 226 with an output port of foot brake valve 224.Delivery port 274 of first solenoid valve 270 is connected in fluidcommunication in line 275 with control port 252 of relay valve 250.

Valve module 200 also includes second solenoid valve 280 having supplyport 281, delivery port 284, and activatable solenoid 286 that iscontrollable in response to electrical control signal on line 234 fromcontroller 210. Supply port 281 of second solenoid valve 280 isconnected through orifice 287 to line 255 to air inlet port 258 that isconnectable in fluid communication in line 230 from compressed airsupply 228. Delivery port 284 of second solenoid valve 280 is connectedin fluid communication in line 275 with control port 252 of relay valve250 and delivery port 274 of first solenoid valve 270.

Valve module 200 further includes one-way check valve 290 having inletport 292 and outlet port 294. Inlet port 292 is connected in fluidcommunication in line 295 with supply port 271 of first solenoid valve270 and air inlet port 262. Outlet port 294 of one-way check valve 290is connected in fluid communication in line 275 with control port 252 ofrelay valve 250 and delivery ports 274, 284 of first and second solenoidvalves 270, 280.

First solenoid valve 270 may comprise an activatable 2/2 valve. Firstsolenoid valve 270 is shown in FIG. 6 in a first position in which airis allowed to flow through first solenoid valve 270 to control port 252of relay valve 250. When solenoid 276 of first solenoid valve 270 isactivated in response to electrical control signal on line 232 fromcontroller 210, first solenoid valve 270 moves from its first positionshown in FIG. 6 to a second position as shown in FIG. 7 in which air isblocked from flowing from foot brake valve 224 through first solenoidvalve 270 to control port 252 of relay valve 250. Spring 278 of firstsolenoid valve 270 returns first solenoid valve 270 from its secondposition shown in FIG. 7 back to its first position shown in FIG. 6 whensolenoid 276 of first solenoid valve 270 is deactivated.

Second solenoid valve 280 may comprise an activatable 2/2 valve. Secondsolenoid valve 280 shown in FIG. 7 is in a first position in which airis blocked from flowing from compressed air supply 228 through secondsolenoid valve 280 to control port 252 of relay valve 250. When solenoid286 of second solenoid valve 280 is activated in response to electricalcontrol signal on line 234 from controller 210, second solenoid valve280 moves from its first position shown in FIG. 7 to a second positionas shown in FIG. 8 in which air is allowed to flow from compressed airsupply 228 through second solenoid valve 280 to control port 252 ofrelay valve 250. Spring 288 of second solenoid valve 280 returns secondsolenoid valve 280 from its second position shown in FIG. 8 back to itsfirst position shown in FIG. 7 when solenoid 286 of second solenoidvalve 280 is deactivated.

Relay valve 250 may comprise an air-operated, graduating directionalcontrol valve of high capacity and fast response. Relay valve 250graduates, holds, and releases air pressure from brake chambers to whichit is connectable.

One-way check valve 290 may comprise any of a variety of types. Outputport 294 of one-way check valve 290 is connected in fluid communicationwith first solenoid valve 270 such that one-way check valve 290 iseffectively connected in parallel with first solenoid valve 270.

Although the above description describes relay valve 250, first solenoidvalve 270, second solenoid valve 280, and one-way check valve 290 asseparate components within valve module 200, it is conceivable that anycombination of these components may be integrated as a single physicalunit.

The arrangement of valve module 200 in the braking system shown in FIG.6 enables air to be controlled and delivered to the one or more servicebrake chambers to affect an autonomous braking event that is auxiliaryto the normal service braking function of the vehicle. Exampleautonomous braking events include hill start assist (HSA), tractioncontrol, electronic stability control, autonomous cruise control (ACC),and collision mitigation. Other types of autonomous braking events arepossible. An autonomous braking event is generally any event where acontroller (such as controller 210 shown in FIG. 6) is controllingcomponents to increase pressure (or maintain pressure in the case of anHSA event) at control port 252 of relay valve 250 without any furtherdriver input (or the driver removes foot from the foot pedal in the caseof an HSA event). For purposes of description, only an HSA event isdescribed in detail hereinbelow.

HSA prevents rolling back of the vehicle when the vehicle is stationaryon an uphill incline and the foot of the vehicle driver is transitioningfrom the foot brake pedal to the foot accelerator pedal to acceleratethe vehicle from its stationary position on the uphill incline.

When the vehicle driver initially depresses the foot brake pedal to stopthe vehicle on an uphill incline, first solenoid valve 270 is in itsfirst position shown in FIG. 6, and second solenoid valve 280 is also inits first position shown in FIG. 6. In the first position of firstsolenoid valve 270 shown in FIG. 6, air flows from foot brake valve 224through first solenoid valve 270 to line 275 to control port 252 ofrelay valve 250. This provides compressed air flow from compressed airsupply 228 through relay valve 250 to line 257 through the one or moreABS modulator valves 240 to operate the one or more service brakechambers 244.

When controller 210 receives a combination of signals including signalon line 212 from foot brake transducer 214, signal on line 216 from oneor more other controllers 218, and signal on line 220 from one or morewheel sensors 222 calling for HSA to be initiated, controller 210provides electrical control signal on line 232 to activate solenoid 276of first solenoid valve 270. When solenoid 276 activates, first solenoidvalve 270 moves from its first position shown in FIG. 6 to the secondposition shown in FIG. 7.

When first solenoid valve 270 is in its second position shown in FIG. 7,air is blocked from flowing from foot brake valve 224 to line 295through first solenoid valve 270 to line 275 to control port 252 ofrelay valve 250. In the second position of second solenoid valve 280shown in FIG. 7, the control pressure previously applied to control port252 of relay 250 becomes “trapped” to hold the pressure in the one ormore service chambers 244 and thereby to maintain application of theservice brakes of the vehicle during HSA.

When first solenoid valve 270 is in its second position shown in FIG. 7,the vehicle driver can increase the control pressure at control port 252of relay valve 250 by depressing the foot brake pedal. When the vehicledriver depresses the foot brake pedal, the pressure in line 295increases, and is passed through one-way check valve 290 to line 275 tocontrol port 252 of relay valve 250. The pressure in line 295 is adriver demand pressure received from foot brake valve 224. Morespecifically, after first solenoid valve 270 has been activated, one-waycheck valve 290 allows for increase of control pressure that is beingapplied to control port 252 of relay valve 250 in response to anincrease of driver demand pressure at foot brake valve 224. The driverdemand pressure from foot brake valve 224 is delivered only throughfirst solenoid valve 270 and not at all through second solenoid valve280 to control port 252 of relay valve 250.

The control pressure applied to control port 252 of relay valve 250 istrapped. The trapped air pressure in line 275 provides HSA, and is notexhausted until first solenoid valve 270 is deactivated and returns backto its first position shown in FIG. 6. In FIG. 6, air trapped at controlport 252 of relay valve 250 is exhausted through first solenoid valve270 to foot brake valve 224 where air is bled to atmosphere.

Second solenoid valve 280 is activatable to move from its first positionshown in FIG. 7 to its second position shown in FIG. 8 to autonomouslyincrease control pressure to control port 252 of relay valve 250. Whensecond solenoid valve 280 is activated and moves from its first positionshown in FIG. 7 to its second position shown in FIG. 8, air supplypressure from compressed air supply 228 passes through second solenoidvalve 280 to control port 252 of relay valve 250. This air pressure fromcompressed air supply 228 is applied to control port 252 of relay valve250 only when no driver demand pressure is being delivered from footbrake valve 224 through first solenoid valve 270 to control port 252 ofrelay valve 250.

Second solenoid valve 280 would be activated only if first solenoidvalve 270 is already activated. Otherwise, compressed air flowingthrough second solenoid valve 280 would exhaust back through firstsolenoid valve 270 and foot brake valve 224. Activation of secondsolenoid valve 280 without first solenoid valve 270 being alreadyactivated is not normal operation of valve module 200. Second solenoidvalve 280 is activated only after first solenoid valve 270 has beenactivated. For example, second solenoid valve 280 can be activated whena small air leak downstream from orifice 287 occurs. As another example,second solenoid valve 280 can be activated when autonomous brakingrequires pressure greater than the driver demand pressure. Orifice 287located between second solenoid valve 280 and compressed air supply 228is provided to reduce the chance of unintentionally applying the servicebrakes of the vehicle in the event that there is a small air leakdownstream from delivery port 284. Orifice 287 allows air to leak outfaster than compressed air from compressed air supply 228 can besupplied.

Referring to FIG. 9, flow diagram 900 depicts a control method for valvemodule 200 of FIGS. 6-8 in accordance with an embodiment. In block 902,a determination is made as to whether an autonomous braking event isdesired. If determination in block 902 is negative (i.e., an autonomousbraking is not desired), the process proceed to block 904. In block 904,both first solenoid valve 270 and second solenoid valve 280 remaindeactivated as shown in their positions in FIG. 6. No pressure isapplied to control port 252 of relay valve 250. However, ifdetermination in block 902 is affirmative (i.e., an autonomous brakingevent is desired), the process proceeds to block 906.

In block 906, first solenoid valve 270 is activated and moves from itsfirst position shown in FIG. 6 to its second position shown in FIG. 7.The process then proceeds to block 908. In block 908, a determination ismade as to whether the desired autonomous braking event has ended. Ifthe determination in block is affirmative (i.e., the autonomous brakingevent has ended), the process proceeds to block 910. In block 910, bothfirst solenoid valve 270 and second solenoid valve 280 are deactivatedand are in their positions shown in FIG. 6. The process returns back toblock 902.

However, if determination in block 908 is negative (i.e., the autonomousbraking event has not ended), then the process proceeds to block 920. Inblock 920, controller 210 determines or updates the desired pressurerequired by controller 210 before proceeding to block 922. In block 922,the pressure being delivered to control port 252 of relay valve 250 andthe desired pressure of controller 210 are compared.

If the pressure being delivered to control port 252 of relay valve 250and the desired pressure of controller 210 are within a predefinedtolerance (e.g., ±5%), then the process proceeds to block 924. In block924, first solenoid valve 270 is activated and second solenoid valve 280is deactivated, as shown in FIG. 7. The process then returns back toblock 908 to continue affecting the autonomous braking event.

If the pressure being delivered to control port 252 of relay valve 250is less than the desired pressure of controller 210, then the processproceeds to block 926. In block 926, first solenoid valve 270 isactivated and second solenoid valve 280 is also activated, as shown inFIG. 8. The process then returns back to block 908 to continue affectingthe autonomous braking event.

If the pressure being delivered to control port 252 of relay valve 250is greater than the driver demand input pressure being received fromfoot brake valve 224, then the process proceeds to block 928. In block928, first solenoid valve 270 is deactivated and second solenoid valve280 is also deactivated, as shown in FIG. 6. When this occurs, thepressure at control port 252 of relay valve 250 is exhausted throughfirst solenoid valve 270 and foot brake valve 224 by bleeding throughfoot brake valve 224. The process loops back to block 908.

It should be apparent that the combination of first solenoid valves 170,270, second solenoid valves 180, 280, one-way check valves 190, 290, andrelay valves 150, 250 in each of valve modules 100, 200, respectively,co-operate to trap brake pressure within the valve module to support anautonomous braking event, such as HSA, of the vehicle.

Although the above description describes valve module 100 of FIG. 1 andvalve module 200 of FIG. 6 being embodied in a vehicle having ABSmodulator valves and a controller that controls the ABS modulatorvalves, it is conceivable that each of valve modules 100, 200 can beembodied in a vehicle that is not equipped with ABS modulator valves. Inthis case, another controller would be used to control air flow througheither valve module 100 of FIG. 1 or valve module 200 of FIG. 6 toservice brake chambers. Accordingly, components within each of valvemodules 100, 200 can be used to control air pressure to provide desiredbrake application pressure without having to rely on ABS modulatorvalves.

A number of advantages are provided by using the arrangement of valvemodule 100 in the braking system shown in FIG. 1 or the arrangement ofvalve module 200 in the braking system shown in FIG. 6. One advantage isthat when air pressure is exhausted from one or more service brakechambers, the air flow is exhausted through only a minimum number ofvalves within valve module 100 of FIG. 1 or valve module 200 of FIG. 6.

More specifically, for valve module 100 of FIG. 1, air pressure from theone or more service brake chambers 144 would exhaust through only firstsolenoid valve 170 and second solenoid valve 180. For valve module 200of FIG. 6, air pressure from service brake chambers 244, 246 wouldexhaust through only first solenoid valve 270 and subsequently bled toatmosphere through foot brake valve 224. In each case, air pressure fromservice brake chambers exhausts through no more than two valves withinthe particular valve module. By exhausting air pressure from servicebrake chambers through no more than two valves, restrictions to air floware minimal. The result is more efficient air flow and, therefore, ahigher performance system for affecting an autonomous braking event.

Another advantage is that each of valve modules 100, 200 requires only arelatively small number of components. Also, the components arenon-complex and are relatively low cost. Accordingly, an autonomousbraking event, such as HSA, is supported at relatively low cost.

Still another advantage is that a modulating control air pressureprovides variable pressure control of autonomous braking events. Avariable pressure control approach to autonomous braking events, such asHSA, avoids the need for a fixed-pressure worst case approach in whichthe highest pressure needed for the maximum grade angle is used. Sincehigh pressure applications are avoided, the life of braking valvecomponents is extended without having to provide more durable brakingvalve components.

Other advantages include improved brake balance and the ability tomodulate the pressure upstream of ABS modulator valves.

While the present disclosure has been illustrated by the description ofexample processes and system components, and while the various processesand components have been described in detail, applicant does not intendto restrict or in any limit the scope of the appended claims to suchdetail. Additional modifications will also readily appear to thoseskilled in the art. The disclosed in its broadest aspects is thereforenot limited to the specific details, implementations, or illustrativeexamples shown and described. Accordingly, departures may be made fromsuch details without departing from the spirit or scope of applicant'sgeneral disclosed concept.

What is claimed is:
 1. A valve module for a vehicle having a compressedair supply, a driver demand device for providing a driver demandpressure indicative of driver intent to apply brakes, one or more brakechambers, and a controller for controlling delivery of air flow throughthe valve module to the one or more brake chambers to control anautonomous braking event of the vehicle, the valve module comprising: arelay valve having a control port, a supply port, and a delivery port,wherein (i) the supply port of the relay valve is connectable in fluidcommunication with the compressed air supply, and (ii) the delivery portof the relay valve is connectable in fluid communication with the one ormore brake chambers; a first solenoid valve having a first solenoid, asupply port, and a delivery port connected in fluid communication withthe control port of the relay valve; and a second solenoid valve havinga second solenoid, a supply port, and a delivery port connected in fluidcommunication with the control port of the relay valve, wherein (i) thefirst solenoid is responsive to a first electrical signal from thecontroller to deliver a first control pressure from the driver demanddevice at the supply port of the first solenoid valve through the firstsolenoid valve to the control port of the relay valve to control airflow from the supply port of the relay valve through the relay valve tothe delivery port of the relay valve to control air flow to the one ormore brake chambers and thereby to control the autonomous braking eventof the vehicle, and (ii) the second solenoid is responsive to a secondelectrical signal from the controller to deliver a second controlpressure control signal from the compressed air supply delivered throughan orifice to the supply port of the second solenoid valve through thesecond solenoid valve to the control port of the relay valve only whenno first control pressure is being delivered from the driver demanddevice through the first solenoid valve to the control port of the relayvalve and (iii) the control port of the relay valve is exhausted throughthe driver demand device in response to the termination of the firstelectrical signal at the first solenoid and the termination of thesecond electrical signal at the second solenoid.
 2. The valve moduleaccording to claim 1, further comprising: a one-way check valve havingan inlet port connectable in fluid communication with the driver demanddevice and being operatively connected in fluid communication with thefirst solenoid valve, wherein: after the first solenoid valve has beenactivated, the one-way check valve allows for increase of the firstcontrol pressure that is being applied to the control port of the relayvalve in response to an increase of driver demand pressure at the driverdemand device.
 3. The valve module according to claim 2, wherein thefirst control pressure control signal from the driver demand device isdelivered only through the first solenoid valve and not at all throughthe second solenoid valve to the control port of the relay valve.
 4. Thevalve module according to claim 3, wherein an output port of the one-waycheck valve is connected in fluid communication with the first solenoidvalve such that the one-way check valve is effectively connected inparallel with the first solenoid valve.
 5. The valve module according toclaim 1, wherein the first solenoid valve comprises an activatable 2/2valve that, when activated in response to the first electrical signalfrom the controller, moves from a first position in which air is allowedto flow through the first solenoid valve to the control port of therelay valve to a second position in which air is blocked from flowingthrough the first solenoid valve to the control port of the relay valve.6. The valve module according to claim 5, wherein the second solenoidvalve comprises an activatable 2/2 valve that, when activated inresponse to the second electrical signal from the controller, moves froma first position in which air is blocked from flowing through the secondsolenoid valve to the control port of the relay valve to a secondposition in which air is allowed to flow through the second solenoidvalve to the control port of the relay valve.
 7. The valve moduleaccording to claim 1, wherein the relay valve comprises an air-operated,graduating directional control valve.
 8. The valve module according toclaim 1, wherein the autonomous braking event comprises hill startassist (HSA).
 9. A valve module for a vehicle having a compressed airsupply, a driver demand device for providing a driver demand inputpressure indicative of driver intent to apply brakes, one or more brakechambers, and a controller for controlling delivery of air flow throughthe valve module to the one or more brake chambers to control anautonomous braking event of the vehicle, the valve module comprising: arelay valve having a control port, a supply port, and a delivery port,wherein (i) the supply port of the relay valve is connectable in fluidcommunication with the compressed air supply, and (ii) the delivery portof the relay valve is connectable in fluid communication with the one ormore brake chambers; a first solenoid valve having a supply port, adelivery port, and a solenoid that is controllable in response to afirst electrical signal from the controller, wherein (i) the supply portof the first solenoid valve is connectable in fluid communication withthe driver demand device, and (ii) the delivery port of the firstsolenoid valve is connected in fluid communication with the control portof the relay valve; a second solenoid valve having a supply port, adelivery port, and a solenoid that is controllable in response to asecond electrical signal from the controller, wherein (i) the supplyport of the second solenoid valve is connectable in fluid communicationthrough an orifice with the compressed air supply, and (ii) the deliveryport of the second solenoid valve is connected in fluid communicationwith the control port of the relay valve and the delivery port of thefirst solenoid valve; and a one-way check valve having an inlet portconnected in fluid communication with the supply port of the firstsolenoid valve and an outlet port connected in fluid communication withthe control port of the relay valve and the delivery ports of the firstand second solenoid valves; wherein (i) the first solenoid valvecomprises an activatable 2/2 valve that, when activated in response tothe first electrical signal from the controller, moves from a firstposition in which air is allowed to flow through the first solenoidvalve to the control port of the relay valve to a second position inwhich air is blocked from flowing through the first solenoid valve tothe control port of the relay valve; (ii) the second solenoid valvecomprises an activatable 2/2 valve that, when activated in response tothe second electrical signal from the controller, moves from a firstposition in which air is blocked from flowing through the secondsolenoid valve to the control port of the relay valve to a secondposition in which air is allowed to flow through the second solenoidvalve to the control port of the relay valve; and (iii) the relay valvecomprises an air-operated, graduating directional control valve; whereinair is exhausted from the control port of the relay valve through thedriver demand device in response to the first solenoid valve and thesecond solenoid valve being deactivated.